There are well-known applications wherein flexible ducts reinforced with reinforcement layers consisting of steel wire are used for carrying fluids, notably hydrocarbons. In some cases, these ducts are placed under conditions where they are subjected to a corrosive environment, for example in the presence of acid fluids comprising sulfur-containing products. Furthermore, in cases where such flexible ducts are placed at great water depths, they must have increasingly high mechanical performances in terms of resistance to the inside pressure, the axial load, the outside pressure due to the great depth of immersion.
In flexible ducts, sealing being provided by one or more polymer sheaths, the mechanical resistance to the inside and outside pressure and to the external mechanical stresses is provided by one or more reinforcement layers consisting of steel wire or sections having a specific profile.
Generally, the flexible tube comprises at least one of the following reinforcement layers: an outside pressure resistance casing made of wire or steel sections arranged at an angle close to 90.degree. to the axis, an inside pressure resistance layer (referred to as pressure layer) arranged at an angle greater than 55.degree., the elongate elements of the casing and of the pressure layer being preferably made of attachable wires, and at least one tensile resistance reinforcement layer arranged with an angle smaller than 55.degree.. According to another method, the pressure layer and the tensile resistance reinforcements are replaced by two symmetrical reinforcement layers at an angle of about 55.degree., or by two pairs of reinforced layers at 55.degree., or by a series of at least two layers, the angle of reinforcement of at least one layer being less than 55.degree. and the angle of reinforcement of at least one other layer being greater than 55.degree.. The steel of the wire forming the reinforcement must be so selected that, considering the cross-section thereof, the wire provides the required mechanical strength during operation while withstanding corrosion, in particular in some cases in the presence of H.sub.2 S.
These steel wires, generally shaped by hot or cold rolling or drawing, can have various profiles or cross-sections: substantially flat or half-flat, U, T or Z-shaped, with or without means for fastening to a neighbouring wire, or circular.
In case of use in the presence of acid gas, mainly H.sub.2 S and CO.sub.2, problems linked with the penetration of hydrogen in the steel can arise in addition to generalized corrosion. In fact, H.sub.2 S (or rather the HS.sup.- ion) is a recombination inhibitor for the hydrogen atoms produced by reduction of the protons at the surface of the steel. These hydrogen atoms enter the metal and recombine therein, thus being at the origin of two types of deterioration:
blisters below the surface of the steel (hydrogen blistering), or internal cracking (referred to as stepwise cracking) that can appear in the absence of stresses and that can be worsened in the presence of residual stresses, PA1 embrittlement resulting in delayed fractures in cases where the steel is subjected to stresses (stress corrosion by hydrogen). PA1 manufacturing a reinforcing wire of sizeable length by rolling or hot wire drawing from steel containing the following elements: PA1 the steel can also contain dispersoids, in particular vanadium, with V.ltoreq.0.1, or possibly V ranging between 0.1 and 0.15 if the wire is not to be welded, PA1 the reinforcing wire having, after being rolled or hot drawn, a temperature at least higher than the AC3 temperature, preferably by 50 to 200.degree. C., and in particular by 100 to 150.degree. C., PA1 winding the wire in reels, and PA1 air cooling of the wire reel to obtain a HRC hardness not less than 40 and preferably greater than or equal to 45, and that can advantageously reach or exceed 50. PA1 it is easy and inexpensive, the wire reel after air hardening can be directly deposited in a drying oven, PA1 the yield limit and the breaking strength are not reduced, the yield limit can even be slightly increased. PA1 0.18% to 0.45% C, preferably 0.20 to 0.40% C, PA1 0.4% to 1.8% Mn, preferably 0.45 to 1.50% Mn, PA1 1 to 4% Cr, preferably 1.5 to 3.5% Cr, PA1 0.1% to 0.6% Si, preferably 0.1 to 0.5% Si, PA1 0 to 1.5% Mo, preferably 0.25 to 1% Mo, PA1 0 to 1.5% Ni, preferably 0 to 0.7% Ni, PA1 at most 0.01% Sand0.020% P, PA1 the steel can also contain dispersoids, in particular vanadium, with V.ltoreq.0.1, or possibly V ranging between 0.1 and 0.15, if the wire is not to be welded.
NACE standards have been provided to assess the ability of a structural steel element to be used in the presence of H.sub.2 S. The steels must be subjected to a test on a representative sample, under stress in an H.sub.2 S medium with a pH value ranging from 2.8 to 3.4 (NACE Test Method TM 0177 relative to the effects of stress corrosion cracking, commonly referred to as Sulfide Stress Corrosion Cracking or SSCC), so as to be able to be considered usable for manufacturing metallic structures withstanding the effects of stress corrosion in the presence of H.sub.2 S.
Another NACE standard (TM 0284) relates to the cracking effects induced by hydrogen, commonly referred to as &lt;&lt;Hydrogen Induced Cracking&gt;&gt; or HIC. The testing procedure recommended by the above-mentioned standard consists in exposing samples, without stress, to a sea-water solution saturated with H.sub.2 S, at ambient temperature and pressure, at a pH value ranging between 4.8 and 5.4. The procedure holds that metallographic examinations are to be performed thereafter in order to quantify cracking of the samples, or to record the absence of cracking.
As subsea reservoir development conditions have become increasingly severe, it recently appeared that material qualification for use in the presence of H.sub.2 S should be aimed at more acid media, since the pH value can be as low as about 3. This has thus led to specify that, in some cases, the tests according to the NACE TM 0284 standard should be carried out in a H.sub.2 S saturated solution with a pH value of 3 or 2.8 for example, similar to the solution defined by the NACE TM 0177 standard, and no longer with a pH value at least equal to 4.8.
According to currently known techniques, the reinforcement wires of flexible pipes, in particular for carrying fluids containing H.sub.2 S, are made with soft or medium carbon-manganese steels (0.15 to 0.50% carbon) with a ferrite-pearlite structure, that are subjected, after cold forming of the rolled wire, to a suitable annealing treatment in order to bring the hardness to the allowed value, if necessary.
The NACE 0175 standard defines that such carbon-manganese steels are compatible with a H.sub.2 S medium if their hardness is less than or equal to 22 HRC.
It has thus been checked that reinforcing wires such as those described above, made of carbon-manganese steel and having a ferrite-pearlite structure, can be manufactured by cold forming followed by annealing so as to meet the conventional NACE criteria. A well-known process described in document FR-A-2,661,194 allows to obtain a steel of hardness higher than 22 HRC compatible with H.sub.2 S according to the NACE TM 0177 and TM 0284 standards, the solution used for the tests according to TM 0284 having a pH value ranging between 4.8 and 5.4.
On the other hand, it has been observed that carbon steels with a ferrite-pearlite structure are incapable of withstanding satisfactorily the HIC tests carried out according to the procedure of the TM 0284 standard when these tests are carried out in a more acid medium, for example with a pH value of the order of 3 corresponding to the conditions henceforth encountered in certain oil reservoir development cases. These unacceptable results are obtained even in cases where the final thermal treatment is more extensive in order to obtain a HRC hardness below 22 HRC.
The manufacture of reinforcing wires for flexible ducts thus requires a steel that is, on the one hand, compatible with H.sub.2 S under the new conditions described above and, on the other hand, that has a relatively conventional and little sophisticated composition and production procedure in order to maintain the manufacturing costs at a sufficiently low level.
Furthermore, the steels and the manufacturing processes used for making reinforcing wires for flexible ducts must be such that the reinforcing wire can be produced in very long continuous lengths of the order of several hundred meters or several kilometers. The wire thus manufactured is wound on reels in order to be used at a later stage to produce the reinforcing layers of the flexible ducts. Besides, and despite the very great unit length of the wires thus produced, it is important that they can be welded together during the reinforcing operation achieved during the manufacture of the flexible duct. In order to restore the specified properties of the steel in the weld zone, in particular resistance to H.sub.2 S, a thermal treatment is to be applied after welding. It is however important, in order not to excessively overload the manufacturing costs, that this thermal treatment after welding allows to reach the objective set within a sufficiently short period of time, of the order of several minutes if possible, preferably less than 30 minutes.
In cases where compatibility with H.sub.2 S is not required &lt;&lt;sweet crude&gt;&gt; production), carbon steels as cold formed also having a ferrite-pearlite structure, but with substantially higher mechanical strength and hardness values, are commonly used. However, it has been observed that the increase in the mechanical strength beyond certain limits leads, for such steels, to an insufficient ductility considering the reforming and reinforcing operations to be achieved with the reinforcing wire.
In the claimant's patent application FR-95/03,093, the reinforcing wire is hardened in a liquid, typically water or oil, which requires high-precision control of the hardening operating conditions and might make wire manufacturing operations more difficult.
The object of the present invention is to describe a process allowing to obtain an elongate element of sizeable length intended for manufacture of flexible tubes, the elongate element having optimized mechanical characteristics and, in an application according to the invention, a good resistance to H.sub.2 S.